Summary: Enteric commensal bacteria impact immune function not only in the intestine, but also at peripheral sites including the lung. In mice, perturbations to the enteric microbiota, whether induced by a ?germ free? state, antibiotics, or dietary modulation, results in altered lung/peripheral immunity. These distant effects include differential susceptibility to Streptococcus pneumonia infection and antibody response to vaccination. Translocated microbial products, such as short chain fatty acids (SCFAs) or capsular components, can signal to immune cell populations at peripheral sites, in some cases systemically priming the innate immune system to enhance pathogen responses. Mechanistic pathways for these distal effects have not been tested or reproduced in humans, neither in healthy adults nor in those with altered immunity as in HIV infection. Indeed, even with effective Antiretroviral Therapy (ART), the incidence of pneumococcal pneumonia and secondary bloodstream infections remain substantially higher in HIV-infected individuals, due in part to compromised pneumococcal-specific antibody response. Potentially contributing to these complications, the gut microbiota of HIV-infected individuals is substantially altered. In this grant proposal, we will investigate the hypothesis that the dysbiotic gut microbiota of HIV-infected subjects contributes to suppressed innate immune activation and phagocytic capacity of Alveolar Macrophage (AM) and enhanced regulation by adaptive immunity, thereby impairing systemic and lung mucosal response to pneumococcal antigen. We will investigate this hypothesis in 3 Specific Aims. In SA1, we will characterize the relationship between gut microbes and their metabolites and immune phenotype and function of AM and T cells in the lung of HIV-infected individuals and HIV-negative controls using an integrated multi?omic approach. We hypothesize that particular gut microbiome compositions and their products/metabolites, will be associated with suppressed activation of pathogen response pathways and phagocytic activity of lung AMs and regulatory phenotypes of both AMs and T cell populations. In SA2, we will investigate the impact of fecal transplantation from HIV-positive subjects on pulmonary immune responses to S. pneumoniae infection using gnotobiotic mice. We hypothesize that fecal transplant of stool from HIV- infected subjects versus controls into gnotobiotic mice alters immune phenotypes in the lung that parallel those in humans and increases severity of S. pneumoniae infection. In SA3, we will examine the relationship between pneumococcal vaccine-specific immune responses in blood and lung of ART-treated subjects and fecal microbiota composition. We hypothesize gut microbiota composition and translocated bacterial products/metabolites wil correlate with pneumococcal vaccine-specific immunity in blood and lung of HIV- infected ART-treated subjects. If an association between the gut microbiome, lung immune phenotypes, and response to pneumococcal infection or vaccination is found, it will open the door towards exploration of microbiome targeted therapies to prevent lung infection and/or increase vaccine response.